This invention relates to molding compositions.
Reaction injection molding (xe2x80x9cRIMxe2x80x9d) is a molding process in which one or more liquid or blending reactants are metered separately to a mixing head which combines them e.g., by high-impingement mixing. The mixture then is injected into a mold where it polymerizes to form a molded part. In structural reaction injection molding (xe2x80x9cSRIMxe2x80x9d), which is often referred to as reinforced reaction injection molding (xe2x80x9cRRIMxe2x80x9d), reinforcements such as chopped glass fiber or particulate mineral fillers are added to the mixture prior to molding. In another type of SRIM process, a low viscosity, partially polymerized RIM composition is injected into a mold filled with woven fiber mat, and the resulting composition molded, In both the RIM and SRIM processes, the molded parts are coated prior to use to provide ultraviolet protection and to match other parts.
A second type of molding process involves premix. Premix is a molding composition prepared prior to and apart from the molding operation which contains all the components necessary for molding, e.g., resin, reinforcing agents, fillers, catalysts, release agents, etc. One type of premix is called sheet molding compound (xe2x80x9cSMCxe2x80x9d). SMC is a thin, semi-tacky sheet of thermosetting resin typically reinforced with chopped or continuous strand glass fibers. The sheet can be molded to form a variety of parts using, e.g., matched die molding techniques. A second type of premix is called bulk molding compound (xe2x80x9cBMCxe2x80x9d). BMC is prepared in the form of a putty that can be directly molded. It can also be extruded in the form of a bar or log to facilitate handling. Like the RIM and SRIM molded products, the molded premix products also are often coated prior to use.
Carbon fibrils are carbon filaments having diameters less than 500 nanometers. Examples of particular carbon fibrils and methods for preparing them are described in the above-referenced Snyder et al. application; Tennent, U.S. Pat. No. 4,663,230 (xe2x80x9cCarbon Fibrils, Method for Producing Same and Compositions Containing Samexe2x80x9d); Tennent et al., U.S. Ser. No. 871,676 filed Jun. 6, 1986 (xe2x80x9cNovel Carbon Fibrils, Method for Producing Same and Compositions Containing Samexe2x80x9d); Tennent et al., U.S. Ser. No. 871,675 filed Jun. 6, 1986 (xe2x80x9cNovel Carbon Fibrils, Method for Producing Same and Encapsulated Catalystxe2x80x9d); Mandeville et al., U.S. Ser. No. 285,817 filed Dec. 16, 1988 (xe2x80x9cFibrilsxe2x80x9d); and McCarthy et al., U.S. Ser. No. 351,967 filed May 15, 1989 (xe2x80x9cSurface Treatment of Carbon Microfibersxe2x80x9d), all of which are assigned to the same assignee as the present application and are hereby incorporated by reference in their entirety.
In general, the invention features a composite that includes a matrix into which carbon fibrils are incorporated, the amount of the fibrils being sufficient to permit the composite to be directly electrostatically overcoated (i.e. without applying a primer coat first).
In one aspect, the composite includes a reaction injection molded matrix into which carbon fibrils have been incorporated.
In a second aspect, the composite includes the molded product of a premix that includes a resin matrix into which carbon fibrils have been incorporated.
In preferred embodiments, the premix is a sheet molding compound or a bulk molding compound. The electrical conductivity of the composite preferably is greater than the electrical conductivity of a composite in which the same matrix is filled with an equivalent amount of carbon black. The amount of fibrils in the composite preferably is sufficient to impart to the composite an electrical conductivity sufficiently high to permit direct electrostatic overcoating. Also preferred are composites in which the amount of fibrils is sufficient to dissipate static electricity. Preferably, the amount is less than or equal to 20% by weight (based on resin), more preferably less than or equal to 4% by weight.
The fibrils preferably are tubes having graphitic layers that are substantially parallel to the fibril axis. One aspect of substantial parallelism is that the projection of the graphite layers on the fibril axis extends for a relatively long distance in terms of the external diameter of the fibril (e.g., at least two fibril diameters, preferably at least five diameters), as described in Snyder et al., U.S. Ser. No. 149,573. These fibrils preferably are also free of a continuous thermal carbon overcoat (i.e. pyrolytically deposited carbon resulting from thermal cracking of the gas feed used to prepare the fibrils). The fibrils preferably have diameters between 3.5 and 75 nanometers, inclusive, and a length to diameter ratio of at least five. Also preferred are fibrils having this morphology in which the outer surface of the graphitic layers is bonded to a plurality of oxygen-containing groups (e.g., a carbonyl, carboxylic acid, carboxylic acid ester, epoxy, vinyl ester, hydroxy, alkoxy, isocyanate, or amide group), or derivatives thereof (e.g., a sulfhydryl, amino, or imino group).
Preferred matrix materials include thermoplastic resins (e.g., polyamide, polyurethane, polyurea, or an elastomer) and thermoset resins (e.g., polydicyclopentadiene, polyester, thermosetting polyurethane, or epoxy resins, or vinylacrylimide resins (such as the Arimix resins commercially available from Ashland Chemical Co., Columbus, Ohio). Resin mixtures may also be used. Either composite preferably is molded in the form of an automotive part for a car, truck, or bus.
In a third aspect, the invention features a composite in a form suitable for reaction injection molding that includes one or more liquid reactants capable of polymerizing to form a reaction injection molded matrix and carbon fibrils.
In a fourth aspect, the invention features a premix that includes a resin into which carbon fibrils are incorporated.
In preferred embodiments, the liquid reactants include one or more polyols, polyisocyanates, or polyamines. The premix preferably is a bulk molding compound or a sheet molding compound. The amount of fibrils is preferably less than or equal to 20% by weight, more preferably lets than or equal to 4% by weight. Preferred fibrils and resins are those described above.
The invention also features methods for preparing the above-described composites.
The reaction injection molded composite is prepared by a method that includes mixing the fibrils with liquid reactants capable of polymerizing to form the matrix; introducing the mixture into a mold; and molding the mixture under reaction conditions including pressure and temperature to prepare the composite in the form of a molded part.
The sheet molding compound composite is prepared by a method that includes mixing the fibrils with a resin and forming the mixture into a sheet. The bulk molding compound composite is prepared by a method that includes mixing the fibrils with a resin to form a putty suitable for molding. Both methods preferably include a molding step in which the composite is prepared in the form of a molded part under reaction conditions that include temperature and pressure.
The molded parts prepared according to the above-described methods are preferably directly electrostatically coated once molding is complete.
The invention provides reaction injection molded composites and molded composites prepared from premix (e.g., sheet molding compound or bulk molding compound) that are electrically conductive at relatively low fibril loadings. This enables molded parts prepared from the composites to be electrostatically coated just as metal parts currently are, thereby eliminating the need for applying a conductive primer coat in a separate application. Further advantages that the fibrils provide include good mechanical properties (e.g., hardness and impact strength) and the ability to use reduced amounts of additives such as flame retardants. The fibrils also provide-inherent EMI shielding.
The use of fibrils offers several processing advantages as well, including good batch to batch consistency with respect to electrical and mechanical properties. In the case of RIM processing, the fibrils, due to their small size, do not plug small lines and orifices in the processing equipment at the low fibril loadings used. Moreover, the fibrils need not become preferentially oriented during processing; thus, they do not contribute to part warpage. In the case of SMC, the increase in viscosity due to the fibrils makes it possible to eliminate thickeners needed to form the tacky sheet.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.
By way of the following examples, the preparation of sheet molding compound (SMC) composites, bulk molding compound (BMC) composites, and reaction injection molded (RIM) composites into which carbon fibrils have been incorporated are described. Preferred fibrils have small diameters (preferably between 3.5 and 75 nanometers), graphitic layers that are substantially parallel to the fibril axis, and are substantially free of a continuous thermal carbon overcoat, as described in Tennent, U.S. Pat. No. 4,663,230; Tennent et al.; U.S. Ser. No. 871,675; Tennent et al., U.S. Ser. No. 871,676; Snyder et al., U.S. Ser. No. 149,573; and Mandeville et al., U.S. Ser. No. 285,817. These fibrils are prepared as described in the aforementioned patent and patent applications. The fibrils may also be treated to introduce oxygen-containing functional groups onto the fibril surface, as described in McCarthy et al., U.S. Ser. No. 351,967.